Christmas Eve in the year 3060, and the planet Puxatornee is home to a prosperous human colony.

A space craft has arrived in orbit carrying the Slithergees, a race of obsequious alien slugs. Their home world has been destroyed and they are humbly requesting permission to settle on the first moon.

And if they donít get permission, then they are humbly threatening to declare all-out war.

The future hangs in the balance. The decision rests with Bailey, the colonyís president -- but she has other things on her mind...

Christmas Eve in the year 3090, and the planet Puxatornee has changed beyond all recognition.

The Doctor and Mel arrive, on a completely unrelated mission to defeat a race of terrible monsters, and soon discover that something rather confusing has been happening to history...

On the White disc, while explaining the hazards of time travel to Mel, the Doctor compares encountering effect before cause to tripping over oneís own footprints. At the equivalent point on the Black disc, while explaining the hazards of time travel to Mel, the Doctor trips over (ahem) something which is never explicity identified.

If the Quarks menacing the space yacht Pinto are the same as those seen in The Dominators, then the natural conclusion would be the yacht is under attack by the Dominators themselves, since the Quarks were no more than their robot servants. However, this is more likely to be a reference to the comic strips of the late í60s, in which the Quarks themselves were touted as a race of evil conquerors. One can always conclude that the Quarks developed self-awareness, overthrew their masters and struck out on their own, but frankly that sort of takes the fun out of it.